Câu d)$cotA+cotB=\frac{cosA}{sinA}+\frac{cosB}{sinB}$$=\frac{sinAcosB+sinBcosA}{sinAsinB}$$=\frac{sin(A+B)}{sinAsinB}$
Câu d)$
\cot
A+
\cot
B=\
dfrac{
\cos
A}{
\sin
A}+\
dfrac{
\cos
B}{
\sin
B}
\\=\frac{
\sin
A
\cos
B+
\sin
B
\cos
A}{
\sin
A
\sin
B}
\\=\
dfrac{
\sin
\left(A+B
\right)}{
\sin
A
\sin
B}$